Re: [TUHS] single user mode in v5 & v6 - TUHS

9 Jan 2015

...
  From: Clem Cole 
...
  ps "knew" about some kernel data structures
and had to compiled with
 the same sources that your kernel used if you want all the command
 field in particular to look reasonable. 
Not just the command field!

The real problem was that all the parameters (e.g. NPROC) were not stored in
the kernel anywhere, so if you wanted to have one copy of the 'ps' binary
which would work on two different machines (but which were running the same
version of the kernel)... rotsa ruck.

I have hacked my V6 to include lines like:

  int     ninode  NINODE;
  int     nfile   NFILE;
  int     nproc   NPROC;

etc so 'ps' can just read those variables to find the table sizes in the
running kernel. (Obviously if you modify a table format, then you're SOL.)

...
  From: Ronald Natalie 
...
  The user structure of the currently running process is
the only one
 that is guaranteed to be in memory ... Any processes that were swapped
 you could read the user structure so things that were stored there were
 often unavailable (particularly the command name).  
Well, 'ps' (even the V6 stock version) was actually prepared to poke around
on the swap device to look at the images of swapped-out processes. And the
command name didn't come from the U area (it wasn't saved there in stock V6),
'ps' actually had to look on the top of the user stack (which is why it
wasn't guaranteed to be accurate - the user process could smash that).

...
  From: Clem cole 
...
  IIRC we had a table of sleep addresses so that ps
could print the
 actual thing you were waiting for not just an address.  
I've hacked my copy of 'ps' to grovel around in the system's symbol
table,
and print 'wchan' symbolically. E.g. here's some of the output of
'ps' on
my system:

 TTY  F  S UID   PID  PPID PRI NIC CPU TIM ADDR  SZ  TXT   WCHAN       COMMAND
 ?:   SL S   0     0     0-100   0  -1 127 1676   16    runout         <swapper>
 ?:    L W   0     1     0  40   0   0 127 1774   43  0 proc+26        /etc/init
 ?:    L W   0    11     1  90   0   0 127 2405   37    tout           /etc/update
 8:    L W   0    12     1  10   0   0 127 2772   72  2 kl11           -
 a:    L W   0    13     1  40   0   0 127 3122   72  2 proc+102       -
 a:    L R   0    22    13 100   0  10   0 3422  138  3                ps axl
 b:    L W   0    14     1  10   0   0 127 2120   41  1 dz11+40        - 4

It's pretty easy to interpret this to see what each process is waiting for.

	Noel